Systems for continuously monitoring blood pressure noninvasively by tonometry have been developed. In such systems, a pressure sensor is held against a portion of the body at which there is a significant artery close to the skin surface. The sensing element is pressed against the skin surface with a sufficient force to partially 15 compress (but not occlude) the underlying artery so that the pressure of the blood within the artery can be sensed by a pressure sensor.
It is desirable to optimize the amount of arterial compression so that accuracy of the pressure reading is assured. In order to so, it has been proposed to provide the sensor with systems for varying the hold-down pressure, that is, the pressure with which the sensing element is pressed against the skin. Such systems are shown in U.S. Pat. Nos. 4,836,213 and 4,987,900, the disclosures of which are incorporated by reference herein. Although such systems are useful for optimizing the sensed pressure pulse, the accuracy of readings can vary from patient to patient, depending upon the amount and compliance of the tissue between the sensor and the underlying artery and on other anatomical variations. Thus, while such a system may provide high accuracy and reliability for a healthy subject whose radius has a typical depression for the radial artery, less than optimum results may be achieved with a subject having an unusually-deep radial depression or who has a significant amount of tissue between the target artery and the skin at the point where the sensor is placed on the body (for example, an obese subject).
In order to overcome these difficulties, sensors have been proposed which incorporate additional systems for moving the sensor element relative to the underlying artery in order to improve pressure sensing. One such arrangement is described in an article entitled "Measurements of Base Blood Pressure During Sleep and Its Clinical Significance in Hypertensive Patients" by Osamu Tochikubo et al in the Japanese Circulation Journal, Vol. 51, October, 1987. Another sensor design employing this feature is shown in Japanese Published Patent Application 2-79904 published on Jun. 20, 1990. While such sensor arrangements may improve the accuracy of blood pressure measurements, these designs are relatively expensive to manufacture and thus are unattractive to be made in disposable form. Furthermore, these proposed sensors have not included provision for use of a multiple-element sensor which requires a very large number of electrical connections.